Final answer:
The suprachiasmatic nucleus (SCN) of the hypothalamus is the brain structure that plays a crucial role in regulating circadian rhythms.
Step-by-step explanation:
The brain structures that play a crucial role in regulating circadian rhythms are located in the hypothalamus, specifically the suprachiasmatic nucleus (SCN). This nucleus acts as the central clock mechanism by setting itself based on light information received through projections from the retina. The SCN sends information to the pineal gland, which is involved in regulating sleep/wake patterns and annual cycles. Therefore, the correct answer to the question "Which brain structures play a crucial role in regulating circadian rhythms?" is a. Suprachiasmatic nucleus (SCN).